In many process installations, a process fluid flows through a conduit, such as process piping. The process fluid may be a liquid, a gas, or a combination of the two. In applications where the process fluid is completely liquid, or completely gas, the calculation of flow parameters, such as mass flow is relatively straightforward. However, in applications where the process fluid is a combination of a liquid and a gas, the calculation of flow parameters is more difficult. Examples of process fluids that generally have a liquid intermingled with a gas (e.g. multiphasic) include wet steam, and natural gas.
In many natural gas well head applications, there is a substantial amount of liquid entrained in the gas stream. The presence of this liquid can cause significant errors in the gas flow measurement. One type of error is sometimes called overreading. Overreading occurs when the differential pressure sensor overreads the true gas flow. The amount of overreading is often correlated to the Lockhart Martinelli parameter, a number that relates the total gas mass flow to the total liquid mass flow. While it may be possible to measure many parameters, and utilize powerful processors to compute flow parameters, such calculations have typically involved dedicated hardware, dedicated technician time and/or testing, or a combination of the two.
Process fluid control installations generally employ process variable transmitters. Providing such a process variable transmitter with the ability to correct for overreading, without adding additional hardware, or involving complex initial characterization or calibration of the device would represent a significant benefit.